Proofreading function of autonomous 3 '-> 5 ' exonucleases in DNA synthesis catalyzed by DNA polymerase beta from rat liver

Autonomous 3'--> 5'-exonucleases (i.e., those not covalently attached to DN A polymerases but often constituting part of replicative complexes) from ra t liver, calf thymus, or Escherichia coil can increase more than ten times the fidelity of DNA polymerase beta from rat liver in reduplication of the primed phi X 174am3 DNA. The degree of correction rises with an increase in the concentration of the 3'--> 5'-exonuclease and reaches two orders of ma gnitude upon extrapolation to the values of the cell activity of the enzyme s of interest. These data cannot be accounted for by the exonuclease degrad ation of primers, as virtually equal results were obtained using 15-nt and 150-nt primers upon measuring the fidelity of DNA polymerases alpha and bet a in the presence of various concentrations of 3'--> 5'-exonuclease. An int ermolecular mechanism is discussed for correcting mistakes in DNA replicati on with participation of autonomous 3'--> 5'-exonucleases capable of splitt ing single-stranded DNA, and moderately processive or distributive DNA poly merases.